Use of bioactive glass for cutting bioactive glasses

ABSTRACT

The present invention relates to a method of cutting a bioactive glass object which comprises contacting the bioactive glass object with bioactive glass particles delivered using an air abrasion system.

This application is a 371 of PCT/GB02/01513 filed on Mar. 28, 2002 whichclaims benefit of 60/281,809 filed on Apr. 6, 2001 abandoned.

The present invention relates to the use of bioactive glasses forcutting and shaping bioactive glasses, particularly bioactive glassimplants and tissue engineering scaffolds.

Bioactive glass implants are beginning to gain a wide acceptance insurgery—e.g. “Douek Med” ossicle replacement grafts, for treating(middle ear disease induced) conductive deafness (Hench L. L., 1998,Bioceramics The Centennial Feature, J. Am. Ceram. Soc., 81, 1705–1728),bioactive glass trans cutaneous and trans osseous abutments for cochlearimplants for middle ear deafness (Hench L. L., 1998, Bioceramics TheCentennial Feature, J. Am. Ceram. Soc., 81, 1705–1728), ERMI—EndosseousRidge Maintenance Implants to conserve jaw bone height after extractionof remaining teeth (Hench L. L., 1998, Bioceramics The CentennialFeature, J. Am. Ceram. Soc., 81, 1705–1728) and orbital floor (eyesocket) repairs after trauma or disease processes (Aitasalo K, SuonpaaJ, Kinnunen I, Yli-Urpo A., 1999, Reconstruction of Orbital floorfractures with bioactive glass (S53P4): In Bioceramics 12. Ed Ogushi H,Hastings G W, Yoshikawa T. World Scientific, London, UK. pp 49–52).

However, all such implants suffer from the same principledeficiency—they are difficult to shape after production to better fitthe patient's individual needs.

Melt derived bioactive glasses are cast into shape using, for example,graphite moulding techniques. Thus bespoke implants can be cast,although undercuts etc in the final form can only be accommodated byinclusion and careful planning of the mould release mechanism.

Sol-gel glass manufacture involves casting a gel in a rigid mould, whichafter controlled desiccation becomes a porous monolithic solid product,approximately 50% of the start volume (Hench L. L., West J. K., 1996,Life Chemistry Reports, 13, 187–241). The gel shrinks in the mouldduring drying and must be freely allowed to contract, otherwise it willtear itself apart during the drying process. Consequently, only thesimplest of product shapes can be produced from the sol-gel process.

There is a great surgical morbidity advantage to be gained by shaping animplant to fit the patient, rather than being forced to adapt apatient's anatomy to match an implant allograft. Therefore, if bioactiveglass materials are to be employed in the surgical sphere forimplantation to aid healing or augment the host's skeletal structure, amechanism for shaping sol-gel glasses and trimming, refining oradjusting the precast melt derived glasses will be highly desirable.

In adjusting the shape of a bioactive glass object, three basic types ofcutting action exist: rotary cutting (e.g. rotating edges chip away atthe substrate as in milling), linear sawing (e.g. a plane action, ordrawn wire saw action) and individual chipping actions (e.g.intermittent chiselling). However, the application of most rotary andlinear cutting techniques to brittle substrates such as bioactive glassobjects inevitably cause fracture, long before the finished productemerges.

Similarly, drilling or boring holes in a bioactive glassobject—necessary for suturing an implant into position—using aconventional cutting technique, such as rotary drilling or sawing, is,at best, extremely problematic.

Rotary and linear cutting techniques generate large amounts of heat dueto the inevitable friction between the cutting surface and thesubstrate. If excessive heat is generated during a cutting process,material can be transferred from the cutting instrument to the finishedproduct surface thereby tainting it. There is therefore a high risk ofpoisoning the delicate bioactive glass reaction systems if rotarycutting is employed. Coolant water sprays decrease cutting temperaturerises dramatically, however bioactive glass materials cannot be cutusing such water sprays and coolants, unless at the immediate point ofuse, as the bioactivity reaction will be started prematurely and theclinical advantage lost.

Air abrasion offers benefits in cutting vulnerable structures such asbioactive glass objects. However, the presence of alumina (oraluminium), the principle abrasive cutting agent in common use today,above a trace level (>1.5 wt %), will totally inhibit or poison thebioactive reaction upon which the bioactive glass implants rely fortheir healing success (Hench L. L., Andersson O., 1993, Bioactiveglasses. In: An Introduction to Bioceramics Chapter 3 pp41–62. Ed: HenchL L, Wilson J World ScientificPub. Singapore and Oonishi H., Hench L.L., Wilson J., Sugihara F., Tsuji E., Kushitani S., Iwaki H., 1999, J.Biomed. Mater. Res., 44, 31–43).

We have now found that by using bioactive glass particles as analternative abrasive agent in a conventional air abrasion system, thebenefits of air abrasion cutting are retained but the problems oftoxicity associated with the use of alumina grit are avoided.

Accordingly the present invention provides a method of cutting abioactive glass object which comprises contacting the bioactive glassobject with bioactive glass particles delivered using an air abrasionsystem.

The present invention is based upon the observation that when appliedthrough a conventional air abrasion system the bioactive glass particlesare able to cut bioactive glass objects to the required shape.

Further advantages arise due to the fact that the bioactive glassparticles and fragments thereof which may become embedded in the cutsurface of the bioactive glass monolith further encouraging rapidre-mineralisation of the affected area and allowing accelerated healing.

The above invention, will allow production of a low internal stressimplant, fitting the patient accurately, and with a surface that ishighly suitable for accelerating bony healing, augmenting existing bonysurfaces and reconstructive surgical procedures (e.g. middle ear ossicleimplants).

To be able to produce more complex shaped products, beyond that possibleby casting alone will prove a significant benefit to the employment ofbioactive glasses in surgical procedures.

DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a schematic of the set-up used in the wire sawcutting experiment, showing the diamond wire (W) reciprocally woundbetween reels (R). The specimen (S) is imaged using long working lengthinternally focussed “Hill” type objectives (O) on a right-angled TSMconfocal microscope (M), using a methylcellulose coupling agent (C). Thesame features are shown in B, the apparatus as set up for real timeconfocal subsurface saw cutting imaging.

FIG. 2-1. A: illustrates a real-time confocal image of 45S5 Bioglass®being sawn. The reflection of the diamond wire saw (SW) is seen, havingproduced an extremely ragged and roughened finish surface (F), fromwhich, several cracks can be seen radiating into the material bulk(arrowed), (Field width 500 μm). B: is an SEM of the same glass type,showing the fractured left margin (M) and lifted unsupported plates ofcavo-surface glass (E). (Field width 2.5 mm).

FIG. 2-2 shows a sawn 58S monolith, showing marked tearing and scoringof the cut surface (S). Cavo-surface angle chipping is pronounced (E)and a fracture is evident, extending off into the bulk of the glass (#).The nearer edge of the slot is missing due to spontaneous fracture (F)from just behind the leading edge of the cut. (Field width ˜1.5 mm).

FIG. 3 illustrates a schematic of the rotary cutting set-up, showingstepper motor (SM) driving the specimen support stage (SS), through alead screw (LS). The support stage, carried on linear ball races, drovethe specimens (SP), aligned with a tripod sub assembly (T) onto therotating bur (B), driven by a step up handpiece (H) (200 kRPM) by motor(M). The confocal microscope's objective lens (OB) was protected bycover slip (CS) from the slot cutting events just beneath. B: shows aclose up image of the specimen carriage (SS) with its sample (opticalplane) adjusting mechanisms, the confocal microscope's objective lens(OB) and the handpiece supporting cradle (C).

FIG. 4-1 illustrates serial real-time confocal images of 45S5 Bioglass®being machined (rotary cut) at A: 0.5 mm/min, B: 1 mm/min, C: 2 mm/min &D: 4 mm/min. In A, there is little cracking and an absence of swarf.However, acceleration of the cutting immediately produced edge chipping(*) and fracture lines (#) began to radiate into the specimen bulk,leading to massive failures (X). Beyond 4 mm/min all specimensshattered. E: shows the remaining part of the same specimen,demonstrating the silicon laminae within the Bioglass® mass andfractures radiating from these planes into the material bulk. F: showsthe view along the cut edge in E to the point of ultimate failurethroughout the monolith, achieved in this specimen at 4 mm/min feedrate. (Field width approx. 1 mm in all cases, except E & F=2 mm).

FIG. 4-2 illustrates real-time Confocal imaging of rotary cutting of 58Sbioactive glass monoliths at A: 0.5 mm/min, B: 1 mm/min, C:2 mm/min &D:4 mm/min. Swarf was produced well at the slower feed rates (S) butmarginal failures (#) occurred at all bar the slowest speeds. Onceestablished, they could not be cut past and fractures (F) radiated intothe bulk of the substrate at higher speeds. (Field widths approx. 1 mm).

FIG. 4-3 illustrates a 58S glass fragment, cut at a maximum feed rate of8 mm/min just prior to failure. A: showing smeared material over theremaining cut face (C) (Field width˜2 mm). Detail of right hand marginB: showed crystalline deposits on the cut glass, confirmed by EDXA tocontain high levels of Tungsten. The fracture plane revealed pore likestructures (P) within the glass mass. (Field width˜300 μm).

FIG. 5 illustrates a schematic of the air abrasion set-up for confocalimaging, showing outer lid (O) clamping the mounting slide (SL) andspecimen (SP) against the inner lid (I) and main structure of the box(Bx), using 3BA brass studding (ST) set into the corner pillars. Bespokegaskets (G) were made from dental silicone impression material. Rubbersheet (R) provided a flexible seal with minimum vibration transmission,for both the air abrader handpiece (H), clamped (C) within the dustchamber and also the vacuum outlet (V). These seals were retained byeither Jubilee type plastic clips (J), or a screw retained tapped steelwasher (W). The Hill type internal focussing long working lengthobjective lens (OB) confocally imaged sub-surface cutting sequences,focussing through the glass mounting slide and clear adhesive. A typicalimage is shown as an insert in part A. Part B shows the set-up mountedon the especially extended stage (X) of the side viewing TSM (T)—hence“X” stage movement became the coarse focus. The cutting events wererecorded via a JAI SIT camera (CA), recording to S-VHS video tape (CookR. J., Azzopardi A., Thompson I. D., Watson T. F., 2001, J. Microsc.,203, 199–207).

FIG. 6 illustrates a schematic diagram showing the air abrasionmachining of slots (#) in substrates (S). The air abrasion nozzle (N)was held in a constant vertical relationship, 3 mm from the uppersurface of the specimen, allowing the abrasive stream (*) to cut a slotin the passing substrate, moved (M) by hand along the metal guide (G).

FIG. 7-1 illustrates air abrasive cutting (LtoR) of 45S5 bioactive glassA–E: ×40 serial real-time confocal images (at approximately 20 msecintervals) of the cutting front, demonstrating re-entrant fractures(arrowed) as the cutting mechanism (Field width˜100 μm: ×40/0.55 nalens). F: SEM of cut surface, clearly showing re-entrant fractures atthe cavo-surface angle (L) and the roughened, amorphous finish surface.No fractures were seen to radiate into the substrate, leaving a soundfinished article (Field width 600 μm). G: High magnification view of thecut face, showing evidence of particulate cutting debris lodged in thecut face, some of which proved to be flakes of Bioglass® (X) and others,alumina (AL)—by EDXA. (Field width 300 μm).

FIG. 7-2 illustrates air abrasive cutting (RtoL) of 58S bioactive glass.A–E: Serial confocal real-time images of the cutting front atapproximately 20 msec intervals, showing serial re-entrant fractures(arrowed) as the mechanism of substrate failure (×24/0.6 na “Hill” lensfield width˜200 μm) F: High magnification view of the finishedcavo-surface angle, showing a well defined margin and the typicalpitted, amorphous finish surface. The porous nature of the cut surfacehas been preserved, not being obstructed by cutting debris (Field width250 μm) G: A low magnification view showing 100 μm sliver of glass beingmachined from the edge of a 58S monolith, without cracking in eithermonolith or cut wafer (Field width 2 mm).

FIG. 8 illustrates two tynes of a 58S sol-gel bioactive glass comb oneabraded with 45S5 bioactive glass (left) and the other with alumina(right), viewed under SEM. Deposition of bioactive cutting particledebris (*) and evidence of impacted alumina residue can be seen on thecut surfaces. Higher magnification of the cutting margin (B), shows anapparently still patent pore network. (Field widths˜4 mm in A & 500 μmin B).

FIG. 9 illustrates the effects of cutting with bioglass abrasive. A:Composite image from one substrate block viewed at a constant distance,showing the effect of progressive separation of air abrader nozzle andtarget (distances shown) at a constant Bioglass® abrasive feed rate andacceleration pressure (80 psi) (Field width˜1 cm). B: Composite imageshowing that the decrease in accelerant pressure reduces cutting depthand thus efficiency, the separation effect also being sustained,regardless of acceleration pressure (Field width˜5 mm).

FIG. 10 illustrates air abrasive cutting (LtoR) of 58S bioactive sol-gelglass using 45S5 Bioglass® particulate as the abrasive material. A–D:×16 serial real-time confocal images (at approximately 20 msecintervals) of the cutting front, demonstrating re-entrant fractures(arrowed) as the cutting mechanism progresses through the substrate(Field width˜400 μm: ×16/0.45 na lens).

FIGS. 11 a and 11 b show the effect of network modifiers (hardening andsoftening agents) and density on glass hardness.

FIG. 12. illustrates a bioactive glass monolith trimmed and with sutureholes (*) bored using bioactive glass air abrasive techniques (A)leaving an intact implant after sterilisation (B). One weekpost-operative Radiographic views show the Lateral Cephalograph (C) andPA skull (D) views. The glass implant (I) sits, tucked behind the bonyinfra orbital rim. There is no evidence of fracture or damage to thebespoke shaped implant mass. The titanium miniplate (M) is a “left over”from the initial surgery 2 years prior to the bioactive implant surgery.

The present invention is suitable for cutting and shaping any bioactiveglass object. Such objects include but are not limited to medical andsurgical implants for human and veterinary use, tissue engineeringscaffolds, drug delivery depots and biosensors. The bioactive glassobjects may be produced either by the melt process or the sol-gelprocess. The bioactive glass objects may be mixtures of more than onebioactive glass.

The term “cutting or cut” as used herein refers to any process wherebymaterial is removed from the bioactive glass substrate, such as carving,shaping, trimming, refining, boring, drilling, finishing, shaving andshearing.

The use of propellant gases or gaseous mixtures other than air (e.g. CO₂or N₂) is included in the definition of “air abrasion” as is the use ofwater or other fluids to act as propellants or as dust suppressionagents—included in the gas stream or entrained around it (e.g. TheAquacut air abrasive machine—Medivance Instruments Ltd, Harlesden,London).

The term “bioactive glass” as used herein refers to a glass or ceramicor material of any particular form e.g. monolithic, foam or otherscaffold formats, comprising Si-oxide or Si-hydroxide which is capableof developing a surface calcium phosphate/hydroxy-carbonate apatitelayer in the presence of an aqueous medium, or at the interface of bodytissues and the glass, so producing a biologically useful response.

Bioactive glass particles suitable for use with the present inventioninclude the silicon based bioactive glasses derived from the Sol-Gelprocess (Hench L L., West J K., 1990, The Sol-gel Process, Chem.Reviews, 90, 33–72) or the Melt process (Hench L L., Wilson J., 1993Introduction to Bioceramics. Publisher: World Scientific). Preferablythe target bioactive glass object is derived from the Sol-Gel process.

Although it may be possible for a bioactive glass lacking a source ofcalcium or phosphorus to generate an apatite layer in vivo by utilisingendogenous sources of these ions, typically a bioactive glass willcomprise a source of at least one of calcium or phosphorous in additionto a source of Si-oxide or Si-hydroxide. Typically the bioactive glasswill comprise a source of calcium. Optionally the bioactive glass maycontain further hardening and/or softening agents. Such softening agentsmay be selected from: sodium, potassium, calcium, magnesium, boron,titanium, aluminum, nitrogen, phosphorous and fluoride. Additions ofsodium, potassium, calcium and phosphorus are most commonly used, toreduce the melting temperature of the glass and to disrupt the Sinetworks within it. Optionally, hardening agents such as TiO₂ may beincluded in the glass composition. Its presence would allowcrystallization to occur within its structure, so producing aglass-ceramic material, whose hardness will be greater than that of theglass alone. An example of a bioactive glass-ceramic material isAppatite/Wollastonite bioactive glass (see Hench L. L., 1998,Bioceramics The Centennial Feature, J. Am. Ceram. Soc., 81, 1705–1728).

Thus, composition ranges for bioactive glasses which may be used withthe present invention are as follow:

SiO₂ or Si(OH)₂ 1–100%  CaO 0–60% P₂O₅ 0–60% Na₂O 0–45% K₂O 0–45% MgO0–40% Plus additions of Na, K, Ca, Mg, B, Ti, Al, P, N and F asnecessary.

The product glasses may contain purely Si/Si gel compounds, or maycomprise two or more of these phases, one of which will be Si/Si gelbased (Bi & tri phasic sol-gel glasses being most commonly used, whereasmelt derived glasses tend to be ternay systems).

Preferably, a bioactive glass will contain between 30 and 100% Si-oxideor Si-hydroxide, more preferably between 40 and 85%.

In a further preferred embodiment the bioactive glass will containbetween 5 and 60% Ca, more preferably between 30 and 55%.

With respect to a source of phosphorus, the bioactive glass will containbetween 5 and 40% P, more preferably between 10 and 30%.

Thus, in one embodiment the bioactive glass particles will compriseSiO₂, CaO and P₂O₅. Preferably the bioactive glass includes from 44 to86 weight % SiO₂, from 4 to 46 weight % CaO and from 3 to 15 weight %P₂O₅. Preferably the bioactive glass is prepared by the sol gel routeand comprises from 55 to 86 weight % SiO₂, from 4 to 33 weight % CaO andfrom 3 to 15 weight % P₂₀₅. Preferably such a bioactive glass has thecomposition 58 weight % SiO₂, 33 weight % CaO and 9 weight % P₂O₅.

In an alternative embodiment the bioactive glass particles may beprepared by the Melt method such as that described in U.S. Pat. No.5,981,412. Such a glass may have a composition of from 40 to 51 weight %SiO₂, 23 to 25 weight % CaO, 23 to 25 weight % Na₂O and 0 to 6 weight %P₂O₅. Preferably such a bioactive glass has the composition (by weight);

-   SiO₂—45%-   NaO₂—24.5%-   CaO—24.5%-   P₂O₅—6%.

Such a bioactive glass is available commercially as Bioglass® 45S5.

The manufacturing and processing methods used in the silicon basedbioactive glass family are ideally suited to the production of tailoredparticles for cutting bioactive glass objects of different strengths andhardnesses.

As mentioned above, hardening and softening components may be added tomodulate the hardness of the bioactive glass particles and hence controlthe cutting action according to the nature of the object glass they areintended to cut. While accepting other known air abrasion cuttingvariables such as particle size, morphology and speed, the greater thedifference in hardness between the glass of the object and the glass ofthe abrasive particles the easier and more efficient the cuttingprocess. In contrast, the smaller the difference in hardness between theglass of the object and the glass of the abrasive particles the slowerand more controllable the cutting process. Thus, either by selectingfrom known bioactive glasses or by varying the amounts of hardeningand/or softening agents present in the abrasive glass particles theskilled man will be able to prepare bioactive glass air abrasive agentscapable of cutting a particular glass object.

Similarly, by controlling the processing conditions in the densificationphase of the sol gel process (Hench L L., West J K., 1990, The Sol-gelProcess, Chem. Reviews, 90, 33–72. Hench L L., West J K., 1996,Biological applications of Bioactive glasses, Life Chemistry Reports,13, 187–241.) sol-gel variants of bioactive glasses can be processed todiffering densities and ultimate strengths and hardnesses.

Vickers Hardness values for exemplary glasses are shown in Table 1. Awell densified 58S sol-gel Bioglass specimen yielded a Vickers Hardnessof approximately 110 (less densified specimens have lower hardnesses)compared with alumina 2,300.

Preferably the abrasive glass particles are at least as hard as that ofthe glass in the object they are intended to cut.

TABLE 1 Vicker's Hardness Numbers. Alumina 2000–2300 Glass beads 500–550Crushed glass powder 500–550 Bioglass ® 45S5 458 +/− 9.4Appatite/Wollastonite bioactive glass-ceramic 680 58S Sol-gel bioactiveglass (fully densified) 110

By increasing the quantity of network modifier (non-silica species, e.g.Na, K, Ca, Mn, Br, Al, N, P, Fl etc) the hardness of the finished glassdecreases. (see FIG. 11 a). These modifiers may be added to the meltderived glasses while in their molten states, or to sol-gel materials atthe mixing phase of production. Hardness may also be decreased byincreasing the porosity within the sol-gel glass, achieved by variationsin the drying and stabilisation and densification phases of theirproduction. As described above, the hardness of glasses can be increasedby allowing crystal formation within them, so the use of TiO₂ can act asa hardening agent, as the glass becomes a glass ceramic. Alsomodifications to the sol-gel processing phases allowing a more denseglass product will result in a harder product (see FIG. 11 b).

A further consideration when preparing a bioactive glass for use in thepresent invention is the size and shape of the bioactive glassparticles. These may be selected depending on the intended application.Angular particles are better suited to cutting quickly through hardmaterials whereas rounded particles are more suited to cutting softermaterials or cutting intricate and precise shapes. The shape ofbioactive glass particles may be controlled by selecting the appropriateparticulation process from, for example, grinding, crushing orair-collision milling during their manufacture. Thus, crushing producessharper angulated particles, whereas, air collision milling will producemore rounded particles. Grinding (e.g. ball milling) however, willproduce particles of a more intermediate shape. Size selection can beachieved with routine sieving processes. These processes are suitablefor glasses produced by both the sol-gel and melt routes.

Particles most suitable for use in the present invention will have adiameter in the range of 1 μm to 1 mm, more preferably in the range of10 μm to 500 μpm.

In cutting a particular object one or more glasses may be employed tocut or shape the object as required.

The present invention may be used with conventional air abrasion systemswell known to those skilled in the art. Examples of suitable airabrasion systems include the Velopex® Alycat marketed by MedivanceInstruments Ltd., which permits switching the source of the abrasiveagent during cutting operations.

It is to be understood that the present invention covers allcombinations of suitable and preferred groups described hereinabove.

The present invention will now be illustrated, but is not intended to belimited, by means of the following examples.

General

Three separate apparatus were developed and constructed to allow realtime imaging and comparison of rotary, linear and air abrasive cuttingof bioactive glasses. Each method is described in turn, to which, fivespecimens of each material test type were submitted. The materialsexamined comprised: 45S5 melt derived Bioglass® objects and 58S sol-gelbioactive glass objects. All material specimens were of a uniform 5 mm×5mm×3 mm deep slab format except in wire saw investigations, where 8 mmdeep slabs were used.

Wire Saw Cutting

Apparatus

The experiments aimed to examine linear saw cutting into the specimentypes, using real-time confocal microscopic imaging of the cuttingprocesses and scanning electron microscopic (SEM) examination of theresidual finished surfaces and margins. A schematic of the experimentalset-up is shown in FIG. 1. The diamond wire saw chosen for the study wasthat considered to be the most “gentle” available within the laboratory.A reciprocating diamond wire saw, (Precision Wire Saw:—Well 3241-2Bennetech, Leicester, UK) specifically designed to section brittlecrystalline materials such as human dentine and enamel. The sawcomprises a fine (approximately 100 μm grit) diamond encrusted stainlesssteel wire, 300 μm diameter & 10 m long. The wire is wound from upper tolower reel and back again, providing the reciprocating action. Theminimum wire velocity of 0.1 m/sec was found to be necessary to avoidbinding or stalling during each pass.

Specimens were held in position on the saw's mounting bracket usingcommand cured dental composite materials (Coltene SE Composite, ColteneWhaledent Dentalvertriebs Gmbh, Konstanz, Germany) and thermoplasticdental composition (Kerr, Romulus, Mich., USA). Each slab specimen (5mm×5 min 8 mm deep) was held with one of its short axes parallel to theline of the wire saw and using the micrometer positioning device, theinitial saw cut was guided to the centre of the 8 mm deep specimen faceoffered to the diamond encrusted wire.

The reciprocating saw machinery was mounted on a cradle on gravityrunway, whose inclination was adjusted to give an applied cutting loadof 10 g—the minimum to ensure free travel of the cradle while keepingthe active wire in constant contact with the substrate. Once alignedwith wire saw and a minimal engaging cut started, the complete sawassembly (mounted on a wheeled trolley) was then brought alongside theTandem Scanning Confocal Reflected Light Microscope—TSM (Koran, Madison,Wis., USA) employed in the study. The illumination for the experimentswas derived from a 100W mercury arc lamp. This instrument had previouslybeen modified for side viewing, allowing in vivo imaging of dentalrestorations using “Hill” type ultra long working length (8 MM)internally focussing lenses (Petroll W., Cavanagh H., Jester J.,Scanning, 1991, vol. 13, I-92 and I-93).

The mounting cradle had been earlier adjusted to deliver mounted glassspecimens to the level of the side-viewing lens. As the saw action wastotally encased within the glass specimen, it was less critical toachieve an absolutely optically flat surface, perpendicular to theoptical axis of the objective lens. Methylcellulose gel (K-Y Jelly. J&JHealthcare, UK) optically coupled the lens to the specimen, furtherreducing the interface surface reflection interference. With gentlerepositioning of the saw machinery and fine adjustments undertaken usingthe lens' own remotely driven internal focus systems (Petroll W.,Cavanagh H., Jester J., Scanning, 1991, vol. 13, I-92 and I-93), dynamicimaging of the wire saw during cutting was possible, image capture beingundertaken using a low light level SIT (Silicon Intensified Target)camera (JAI, Copenhagen, Denmark.), recording to S-VHS videotape.Real-time sequences (25 frames per second) of particular note andspecific frames of interest were later converted to digital format,using a Studio MP10 converter (Pinnacle Systems, Calif., USA).

Short video sequences of cutting were found to be recordable, only inthe more translucent specimens. The opacity of the 58S glass defeatedthe confocal system and video imaging was not possible with so littlecontrast from such high-speed events at any significant depth within thesol-gel glass substrate. Real-time in-situ images were therefore onlyrecorded for the clear 45S5 bioactive glass materials. All cut specimenswere however retained for SEM imaging and cut surface/edge analysis andinterpretation afterwards.

Results

45S5 Bioactive Glass—(FIG. 2-1)

Being reasonably hard and brittle, 45S5 Bioglass® displayed bothchipping and fracture processes both superficially and deep within thebulk of the saw cut itself, during each cutting stroke. The images fromboth SEM and real-time confocal imaging, showed evidence of similarfracture—failure cutting patterns, the leading edge of the glass beingparticularly vulnerable to cracks extending at least a short way intothe mass of the material. If close to a second surface, substratefailure would occur, the crack propagating toward that surface.Typically one marginal wall would fracture out to the bulk's surface orunsupported cavo-surface angles and edges failed, plates of superficialglass lifting at the cavo-surface margins.

The size of fractured plates of glass lifted from the cavo-surface anglewas of the order of 100 μm diameter each. Furthermore, it provedimpossible to identify the saw blade's pattern of movement in thismaterial as no evidence of slumping or thermoplastic behaviour wasdiscernable from the scored and cracked finish surface patterns.

The findings reflect the interaction of a hard brittle substrate withthe individual diamond crystals of the fine wire saw, little of theresidual surface morphology being attributable to thermally generatedfracture.

58S Sol-Gel Bioactive Glass—(FIG. 2-2)

The 58S bioactive glass specimens frequently suffered catastrophicfailure, large pieces (several millimeters wide) fracturing away fromthe sawn line. Fractures radiated off from the saw path and unsupportedcavo-surface edges were also prone to localised chipping and fracturetoo. The sawn surface showed deep, ragged, gouges and striations,similar to that seen in the 45S5 specimens. Similarly, the direction ofthe wire saw's movement could not be positively interpreted from the SEMimages.

The substrate bulk fractures, often arose from the greatest depths ofthe sawn slot, giving the impression that even if seemingly sound at theend of a cutting phase, the specimen was likely to be fundamentallyflawed and weakened. Indeed in some specimens the failures occurredspontaneously from within the bulk of the substrate. Although confocalimaging was not possible, the similarity of 58S and 45S5 glass SEMimages and their concordance with the confocal real-time images showingfractures extending off into the cut substrate bulk, supports theassertion that similar behaviour is occurring in both of these bioactiveglasses.

Thus, glass substrate fracture was commonly seen for both the sol andmelt derived glasses. Thus wire saw cutting is inappropriate for cuttingand shaping bioactive glass objects shaping.

Rotary Cutting

Apparatus

A precision sliding carriage microscope sub-stage, originally designedand constructed for dental cutting experiments (Watson T., Flannagan D.,Stone D., B.D.J., 2000, vol. 157, p 680–686, Watson T., Cook R., 1995,J. Dent. Res., 74, 1749–1755 and Watson T., 1990, J. Microsc., vol. 157,p 51–60) was adapted to allow imaging of the rotary cutting of glassmaterials. A schematic of the experimental setup is shown in FIG. 3.Test specimens, mounted on a bespoke jig, using light cured dentalcomposite resin as an auto-casting, command set rigid support medium(Coltene SE Composite, Coltene Whaledent, Dentalvertriebs Gmbh,Konstanz, Germany), were introduced to a rotating cutting bur whose longaxis was aligned with the conventional vertical optical axis of a TandemScanning Confocal Reflected Light Microscope—the TSM (Noran, Madison,Wis., USA) with 100W mercury arc illumination and long working rangeobjective lenses.

Using a glass coverslip as a guide, the upper surface of theexperimental materials were contrived to be par-focal with the endcutting flutes of the bur and were held in an optically flat plane,judged by the phenomenon of chromatic aberration (Watson T., 1997, Adv.Dent. Res., vol. 11, p 433–441 and Watson T., Cook R., 1995, J. Dent.Res., 74, 1749–1755).

The specimen support jig was itself bolted to an intermediate tripodframe on the specimen carriage, allowing adjustment of the specimen'supper surface in three planes. Although viewing sub surface eventsduring the cutting to avoid inclusion of erroneous unsupported surfacefailure patterns, it was essential that the specimen be held level withthe cutting bur end, to avoid confusing side-cutting events with theproblematical phenomena of end cutting (Watson T., Cook R., 1995, J.Dent. Res., 74, 1749–1755).

It is well accepted that the more concentric a rotary cuttinginstrument, the less heat, vibration and unwanted side effects willarise during cutting (Watson T., Cook R., 1995, J. Dent. Res., 74,1749–1755). For this reason, the crystalline irregularities of diamondburs were rejected in favour of the most concentric (i.e. one-pieceengineered, non cross cut fissure pattern) tungsten carbide bursavailable (“Smartburs” Precision Rotary Instruments Inc, BridgewaterCorners, Vt., USA) (Watson T., Cook R., 1995, J. Dent. Res., 74,1749–1755). This particular brand had previously been demonstrated tohave significant advantages over typical two part (T.C. head sintered tosteel shank) burs, especially in the condition of residual substratewhen cutting hard, brittle materials (Watson T., Flannagan D., Stone D.,B.D.J., 2000, vol. 157, p 680–686, Watson T., Cook R., 1995, J. Dent.Res., 74, 1749–1755). A Fresh bur was made available for each cuttingaction in each specimen group, so making the comparison as fair aspossible.

Thus, real time confocal reflected light imaging of the substrate'sstructure being cut, was achieved (Watson T., Flannagan D., Stone D.,B.D.J., 2000, vol. 157, p 680–686, Watson T., Cook R., 1995, J. Dent.Res., 74, 1749–1755 and Watson T., 1990, J. Microsc., vol. 157, p51–60). The long axis of the rotating bur did not change during eachpass, allowing the point of cutting to be imaged throughout individualexperiments, as the specimen was carried forwards onto the rotary bur bya calibrated, stepper motor driven lead screw. Thus effectively, a slotmachining process was imaged in real time, the advance rate of thespecimen being known and recorded throughout the procedure (0.5–8 mm/minadvance rate range). The images were captured using a cooled ChargedCoupled Device (CCD) monochrome camera (Cohu), through the confocalmicroscope's imaging port, and were stored on S-VHS video tape for lateranalysis. An audio commentary provided synchronised cutting speed/timedata during playback.

All rotary cut specimens were imaged during their cutting, and thespecimens were retrieved afterwards, being submitted for SEM examinationof their cut surfaces and edges. Where substrates failed, the fragmentswere collected as best possible, and were nonetheless submitted for SEMexamination.

Results.

45S5 Bioglass®—(FIG. 4-1)

During each experiment on each Bioglass monolith, fracture of thesubstrate was always seen both local to the cutting process itself andradiating off into the bulk of the substrate. Even at the minimal feedrate of 0.5–1 mm/minute, spontaneous fractures were seen to propagateinto the substrate mass. The poor cutting was matched by a poor swarfproduction. Those few particles seen to develop, appeared as smallsplinters rather than aggregated material often seen in the cutting ofmachineable borosilicate glasses.

At any appreciable cutting rate, the Bioglass® structure failed en mass.Retrieved sections—usually the better supported entry side (left side inthe real time cutting images, as the bur rotated clockwise from theviewing perspective)—showed no evidence of rippling and at no time wasbur uprighting seen, implying that there was minimal vibration in thecutting system. However, examination of the cut surfaces revealed theshattered edges of the silicon laminae, individually failing andproducing a series of steps in the Bioglass® finish surface.

58S Sol-Gel Bioactive Glass—(FIGS. 4-2 & 4-3)

The sol-gel specimens behaved very differently to melt derived glasseswhen subjected to rotary cutting. At very slow feed rates (0.5 mm/min) agood aggregated swarf was produced (resembling that of the machineableborosilicate glasses, in shape and production rate), but on theunsupported exit side of the machined slot, substrate failure wasregularly seen. At slow advance speeds (up to 1 mm/min) the failureappeared as localised fracture or “crumbling” of the marginal glass:Once established, an exit side failure could not be cut past i.e. as thebur approached the deepest extent of the previous failure, rather thanestablish a new slot margin, further exit side collapse wouldpreferentially occur, perpetuating the cycle. At higher feed rates, theexit side failure pattern was perpetuated but the fragments lost werelarger at each event, reflecting a greater energy input at highercutting speeds. Beyond 4 mm/min, all specimens shattered and were lost,only small fragments remaining. Examination of an entry side fragmentclearly showed multiple fractures radiating into the substrate from thecut surface. Fractures were not seen on this side during the real-timeimaging of the cutting events, as the bulk material was self-supportingon the entry side. It was noted that just prior to failure, paleincandescence was visible from some of the specimens, accompanied by adeterioration in the contrast of the confocal cutting image, caused byadditional light entering the confocal system, generated from within theoptical focal plane itself.

This alone indicated an excessive amount of heat being generated at thecutting interface, enough to cause transfer of tungsten metal crystalsfrom bur to substrate surface (confirmed on SEM examination of theresidual cut glass specimens) Bearing in mind the fragility of thebioactive reaction, such adulteration of the cut glass surface isunacceptable for both experimental and medical use.

Examination of the Tungsten carbide Smartburs after machining one 5 mmslot in a single 58S monolith revealed the amount of wear induced inthis one cutting action. The transfer of metal to the residual substrateand the loss of all its sharp working surfaces and edge profiles,providing further evidence of the enormous heat and abrasive damageexperienced during a single cutting process.

Thus, rotary cutting is hopeless for any shaping process for sol-gelglasses, while only the most superficial trimming of surfaces of largemonoliths of 45S5 glass may be practical. Any fine surface detail willnot survive, despite using the most concentric cutting instrumentsavailable (Watson T., Cook R., 1995, J. Dent. Res., 74, 1749–1755).

Alumina Air Abrasion Cutting

Apparatus

A schematic of the experimental setup is shown in FIG. 5. The apparatusdesign (Cook R. J., Azzopardi A., Thompson I. D., Watson T. F., 2001,The cutting Edge of Air Abrasion. Procs Far East Asia Second Symposiumon Confocal Microscopy Sun-Yat Sen University, Taiwan. In Press and CookR. J., Azzopardi A., Thompson I. D., Watson T. F., 2001, J. Microsc.,203, 199–207) comprised a rigid dust containment chamber, whose internalpressure was maintained below that of atmosphere by entraining airthrough all breaches in the walls' integrity, thereby minimisingabrasive escape. The partial vacuum and unidirectional airflow wasmaintained using the commercial dust handling vacuum cleaner, suppliedwith the Lares air abrasion cutting unit employed (Lares Industries,California, USA).

The basic chamber consisted of a simple box: 20×35×5 cm constructed fromsawn 5 mm thick sheet Perspex®. All joints were cemented usingchloroform as a solvent and the corners were internally reinforced with2×1 cm Perspex® pillars, rebated to allow the internally fitting firstlid to rest on their upper surface and remain a flush fit with thewalls. The lid, drilled to match, was retained using brass nuts oncaptured 3BA thread brass studding, set permanently in the axiallydrilled pillars, using a dissolved Perspex®/chloroform slurry.

The baseplate was drilled to allow access for the vacuum coupling, madefrom a spare 35 mm photographic film plastic container. To minimizetransmitted vibrations from the vacuum apparatus via its hose, a softflexible connector was produced to bridge the gap, by folding a sheet ofdental rubber dam into a cylinder and securing it to both the chamberoutlet and the vacuum hose, with “Jubilee” type clips.

Under test vacuum, the lid was found to flex, so Perspex legs werefitted, providing central support. Similarly, a rigid holding clamp wasconstructed for the air abrader hand-piece within the cell. The airabrader hand-piece access hole was closed using a gasket of dental“Rubber dam,” retained by a large steel “O” washer, tapped peripherally.The flexible sheet allowed adjustment of the hand-piece position withinthe cell, while maintaining a dust-tight seal.

In service, it was found that several 5 mm sidewall access holes(allowing screwdriver access for internal adjustments) were welltolerated by the system. Despite air abrasion inflow pressures of up to100 psi, a relative internal vacuum, approximately 5–7 mmHg less thanatmosphere, was successfully maintained, as the inflow volumes wererelatively low.

The inner lid had a centrally cut access window, with dimensions 4 mmsmaller than a standard microscope slide, so allowing mounting of thespecimens within the chamber, and alignment of the air abrader nozzle 3mm away from and perpendicular to the facing surface of the specimen.The glass microscope slide, to which the specimens were adhered, usingthe thinnest possible film of low viscosity clear cyanoacrylate impactadhesive (Watson T., Pilecki P., 1999, Procs. RMS, vol. 34. pp 485–487)was held in place by a second outer lid of 3 mm clear polycarbonatesheet, clamped by the corner fixing brass stud/nuts described above. Toallow imaging, this too had a central viewing window matching that ofthe inner lid. To hold the specimen preparation in place and ensure aseal close to the optics, addition cured silicone dental impressionmaterials (Aquasil, Dentsply DeTrey GmbH, Germany) were sparingly placedat the lid margins, producing bespoke gaskets.

The design thus allowed viewing of each specimen through a fresh glasswindow and the flexible hand-piece seal allowed re-alignments andseveral cutting attempts from each of the five specimens of eachmaterial group. During higher power confocal imaging, it was founduseful to flat polish the air abrader head, allowing the nozzle orificeto come within 1 mm of the glass to maximise the depth of focus into thecut area of the substrate.

The chamber was securely bolted to an “X” axis extension platformfabricated to fit the conventional “X-Y” stage. Therefore, the stagecontrols were reassigned, the conventional “X” became the coarse “Z”focus, “Y” became the new “X” and the original “Z” became “Y”. Thisadaptation though not essential, allowed handpiece and vacuum accessthrough the baseplate (See FIG. 5). Fine focus was achieved usinginternally focussing long working length, “Hill” pattern (Petroll et al1991) objective lenses: ×16/0.45 nd, ×24/0.6 nd (Tandem Scanning Corp.Annapolis, Md., USA) and a dry ×40/0.55 na lens (Nikon, Japan). Specimenimaging through the microscope slide and low viscosity clearcyanoacrylate impact adhesive was therefore straightforward. Prior tobeing mounted, specimens not having a finished flat surface were handpolished to P1200 grit, so minimising the adhesive interface depth, astrials had shown superior imaging of internal cutting events by thismethod.

All cutting experiments were undertaken using the same Lares (LaresIndustries, California, USA) air abrasion machine incorporating a 600 μmdiameter cutting nozzle, 27 μm diameter alumina cutting particulate andan instrument acceleration pressure setting of 80 psi. The same initialtarget to nozzle separation of 3 mm was also maintained throughout,allowing direct comparison of results, although imaging was fromdifferent depths within the specimens. Likewise, the same ‘medium’powder flow rate (0.01 g/sec) was employed throughout.

To allow real time direct reflection imaging of the cuttinginteractions, a Tandem Scanning confocal Microscope—TSM (Noran, Madison,Wis., USA) was employed with 100W mercury arc illumination. Thisinstrument had previously been modified for side viewing, allowing invivo imaging of dental restorations. Image capture was undertaken usinga low light level SIT (silicon intensified target) camera (JAI,Copenhagen, Denmark.), recording to S-VHS videotape. Real time sequencesof particular note were later converted to digital format, using aStudio MP10 converter (Pinnacle Systems, California, USA), also allowingabstraction of specific frames of interest for illustrative purposes.

Cutting was thus imaged in real time, but little SEM evidence waspossible unless the cyanoacrylate adhesive interface could be persuadedto fail at the end of the cutting run. Even so, the upper surfaces ofthe specimens were often found to have an adhesive coating, maskingpossible fractures or features within the specimen material beneath.Furthermore, any surface irregularities found, could have arisen fromthe de-bonding process.

Consequently, a final group of specimens were subjected to air abrasivecutting of a slot, machined in the same orientation as employed in therotary and linear cutting work. A schematic of this experimental setupis shown in FIG. 6. The specimen was held flat on the floor of acommercial abrasive containment chamber (Handler, USA) and a slot wasmachined vertically downwards through the glass slabs, using a contrivedjig/rest to maintain a constant nozzle—target distance of threemillimeters as above. Machining a linear slot was achieved by moving theglass slab along a straight metal edge within the dust hood, passing theglass beneath the vertically orientated air abrasive nozzle. The sameacceleration Pressure (80 psi), powder flow rate (0.01 g/sec) and thesame 27 μm alumina particle diameter was maintained throughout, allowingdirect comparison of results.

Results

45S5 Bioglass®—(FIG. 7-1)

45S5 bioactive glass monoliths were cut extremely easily and well usingthis system. Recurrent re-entry patterns of fracture were seen duringthe real-time image analyses performed after each cutting action. Unlikethe rotary cutting finished surface patterns, there was no evidence ofthe pattern of silicon laminae within the glass—rather, a roughenedamorphous surface, with a well defined but rounded cavo-surface angle.No evidence was seen of fractures radiating into the bulk substrate.

The cut surfaces appeared clean on first inspection but traces ofalumina (small, (sub)-micron proportioned particles, presumably leftafter abrasive particles impacted and perhaps shattered), were foundusing energy dispersive spectra (EDXA) on SEM examination of residualsurfaces.

58S Sol-Gel Bioactive Glass—(FIG. 7-2)

58S sol-gel bioactive glass cut equally well and brittle substratefailure occurred in exactly the same manner as described above for the45S5 Bioglass®. Similarly well-delineated but rounded cavo-surfaceangles at both entry and exit sides were found and delaminating flakesof swarf were again identified during pauses in the cutting activity. Nocatastrophic specimen fractures were identified during any cuttingsequence.

The sol-gel glass monolith is a porous matrix, unlike melt derived 45S5bioactive glass. On first inspection of the residual cut surfaces, asimilarly amorphous, chipped and roughened surface was apparent. Closersurface inspection suggested the inherent pore matrix may still bepatent, the concern being that cutting debris obstruction (akin to smearlayers when rotary cutting dentine) would drastically reduce tissuefluid accessible glass reaction interstices, drastically altering theglass monolith's bioactive reaction kinetics.

This work confirmed the previously hypothesised highly localised brittlepattern re-entrant fracture theory (Horiguchi S., Yamada T., InokoshiS., Tagami J., 1998, Operative Dentistry, 23, 236–243) by directobservations on brittle substrates in real time, further supported byimaging the swarf and residual surfaces seen during and just aftercutting (Cook R. J., Azzopardi A., Thompson I. D., Watson T. F., 2001,The cutting Edge of Air Abrasion. Procs Far East Asia Second Symposiumon Confocal Microscopy Sun-Yat Sen University, Taiwan. In Press and CookR. J., Azzopardi A., Thompson I. D., Watson T. F., 2001, J. Microsc.,203, 199–207). The kinetic energy transferred from each alumina particleimpact is minimal compared with either the overall target mass or thecrude blades and embedded crystals of typical rotary cuttinginstruments, so minimising the likelihood of catastrophic substratefailure during machining.

Bioglass Air Abrasion Cutting (1)

Apparatus

The open jig alignment apparatus referred to above (FIG. 5), wasemployed in these experiments. One side of a series of five 58S sol-gelmonoliths (1 cm×1 cm×3 mm) were subjected to a 5 second cutting actionusing an alumina abrasive, while the remaining sides were subjected toan equivalent duration of air abrasion, using similar size range (20–90μm) 45S5 Bioglass® particles, accelerated at the same 80 psi pressureand projected through the same 600 μm diameter nozzle, at a nominal 5 nmrange of nozzle to target surface, under constant delivery rate of 0.01g/sec. The two sides of each test substrate specimen were isolated usinga razor blade, protecting each surface from the effects of thealternative treatment. All air abrasion cutting activities wereconducted within a purpose built, self evacuating chamber to minimiseenvironmental pollution (Handler, Westfield, N.J., USA).

Between abrasive treatments, and after allowing 2 mins of passing cleanair to clear residual cutting abrasive from the system, no particulatewas detected in the cutting air stream and the abrasive was changed fromalumina to the 45S5 Bioglass powder. The resulting cut specimens wereblown clean with compressed air and were committed for carbon coatingand SEM examination to allow characterisation and comparison of theresidual finished surfaces.

To provide a complete characterisation of the air abrasive cuttingprocess as applied to 58S sol-gel bioactive glasses and to compareimpact patterns with those already known for alumina, additional 58Smonoliths were exposed firstly to perpendicular 3 second bursts ofBioglass® abrasive at a constant acceleration pressure of 80 psi, butwith increasing nozzle-target distances (0–8 mm), then subsequently, theacceleration pressure was decreased to 60 psi at 2 & 4 mm ranges. Thetreated 58S surfaces were subjected to SEM examination to demonstratethe margins and cutting depths achieved by both acceleration pressureand range.

Results

FIG. 8 demonstrates that while not cutting so rapidly as aluminaparticles, the Bioglass® abrasive did achieve a very distinct cuttingaction against the sol-gel substrate. 45S5 bioglass particles areembedded in/on the finished surface as were the more angular aluminacutting particles—indeed a plug of alumina was left in the cut well ofthe representative sample imaged. Close examination of the Bioglass®air-abrasion finish surface showed an apparently retained open porenetwork and the classically amorphous, chipped finished surfacecharacteristic of air abrasion cutting (Goldstein R., Parkins F., 1994,J. Am. Dent. Assn., 125, 551–557, Laurell K A, Hess J., 1995,Quintessence, 26 (2), 139–143, Banerjee A., Kidd E., Watson T., 2000, J.Dentistry, 28, 179–186, Cook R. J., Azzopardi A., Thompson I. D., WatsonT. F., 2001, The cutting Edge of Air Abrasion. Procs Far East AsiaSecond Symposium on Confocal Microscopy Sun-Yat Sen University, Taiwan.In Press, Cook R. J., Azzopardi A., Thompson I. D., Watson T. F., 2001,J. Microsc., 203, 199–207). When compared to the obviously poisonedsurfaces from rotary cutting experiments the advantages of this cuttingtechnique are clear.

The results of the second phase of this experiment (FIG. 9) confirmedthat with reduced pressures and increased distances of the nozzle fromthe target surface, the cavo-surface angle of the cut bore becomes moreobtuse, less well defined and the depth of the cut decreases.

The kinetic energy transferred from each Bioglass® particle impact isminimal compared with either the overall target mass or the crude bladesand embedded crystals of typical rotary cutting instruments, sominimising the likelihood of catastrophic substrate failure duringmachining. By way of confirmation, no catastrophic specimen fractureswere identified during any cutting sequence, thus commending the airabrasion method for accurately dry cutting and shaping difficult,vulnerable, brittle, moisture and heat sensitive materials, leavingrounded stress lowering margins, ideal for brittle materials and alsowith regard to the bioactivity of the residual surfaces, untainted byalumina fragments and cutting debris.

Bioglass Air Abrasion Cutting (2)

Apparatus

Using the same experimental design, equipment and set up as used in theAlumina air abrasion cutting above, but substituting a similar sizerange particulate of 45S5 Bioglass for the more conventional 27 μm gritalumina particulate, specimens of 58S sol-gel substrate glass weresubjected to air abrasive cutting and real-time confocal imaging in theconventional way described.

Results—(FIG. 10)

The confocal images in FIG. 10 confirm by their similarity to those inFIGS. 7-1 and 7-2 that the cutting mechanism of one bioactive glassagainst another is fundamentally the same as that applicable to aluminaabrasive cutting into the same brittle substrate. Re-entrant fracturescan be seen at the cutting edge, whose advance rate, although rapid, wasnot quite as efficient as if alumina had been used. The 45S5 particlesused were more rounded in shape than the alumina, which along with theirlower hardness, would account for this result. However, useful, accuratecutting has been demonstrated, without the risk of residual surfacetoxicity.

By observing the cut edges and the nature of the walls of the holesproduced, just behind the active cutting face, this method has indicatedthat the purely end cutting process, just like that seen in the aluminacutting work undertaken by Cook et al (Cook R. J., Azzopardi A.,Thompson I. D., Watson T. F., 2001, The cutting Edge of Air Abrasion.Procs Far East Asia Second Symposium on Confocal Microscopy Sun-Yat SenUniversity, Taiwan. In Press and Cook R. J., Azzopardi A., Thompson I.D., Watson T. F., 2001, J. Microsc., 203, 199–207), also applies to theBioglass® cutting particulates too, as the identical surfacemorphologies and cutting behaviours are seen in both groups.

It would appear to be more efficient to machine a slot or reduce asurface level by making multiple passes over the target, rather than toachieve a finished depth and then to move laterally, as the cuttingmechanism is a principally end-cutting phenomenon (Cook R. J., AzzopardiA., Thompson I. D., Watson T. F., 2001, The cutting Edge of AirAbrasion. Procs Far East Asia Second Symposium on Confocal MicroscopySun-Yat Sen University, Taiwan. In Press and Cook R. J., Azzopardi A.,Thompson I. D., Watson T. F., 2001, J. Microsc., 203, 199–207). Anyopening out will allow adequate exhausting to be established andmaintained. Successive passes may be made equally efficient by advancingthe airbrader nozzle toward the target, maintaining an optimalseparation. More rounded finish contours can simply be achieved byincreasing the separation of nozzle and target. Flattened slit like airabrasive nozzles have been manufactured in the past and were reportedlycapable of cutting extremely fine slots, or even sectioning materialsfor microscopy (Boyde A., J. Dent. Res., 1963, vol. 42, p 1115).However, the round orifice is most likely to be chosen forgeneral-purpose work, as this allows any shape of cavity to be machinedwith least operator concern over varying orientations of the nozzle axesand the target.

Clinical Application (FIG. 12)

After the failure of conventional therapy methods for post traumaticorbital volume increase, a decision was taken to reconstruct the floorof a patient's eye socket with a 45S5 bioactive glass monolith implant.Pre-operatively, a series of 5 custom made bioactive glass monolithswere conventionally cast using a bespoke graphite mould and plugtechnique. The mould shape was established by hand copying profiles froma dried human skull for the superior contours, the known volume deficitwithin the patient's damaged orbit and inferior contours beinginterpreted from the CT information. Glass monoliths of approximately2×3.5 cm and varying from 4–8 mm depth (mean volume=4.2 ml), were thusproduced (See FIG. 12).

After casting, further monolith shaping was successfully conducted,using the air abrasion cutting technology described above. Suture holeswere then bored (See FIG. 12) also using the bioactive glass airabrasion technique. The monoliths were then subjected to routinepre-operative hospital standard sterilising processes.

Surgical access to the patient's orbital floor was achieved via apre-existing infra-orbital incision, and the sub-periosteal implant wasfirmly sutured into place using un-dyed 3/0 Vicryl® suture, passingthrough both the implant holes provided and a pair of small bur holes inthe inferior bony orbital rim of the patient.

Radiographic examination confirmed accurate placement and stability ofthe Bioglass® implant, while also revealing how good the implant—bonefit was (See FIG. 12). Follow up, at six months after placement,confirmed the total success of the procedure and the stability andsustained integrity of the shape of the implant.

1. A method of cutting a bioactive glass object which comprisescontacting the bioactive glass object with bioactive glass particlesdelivered using an air abrasion system.
 2. A method of cutting abioactive glass object which comprises contacting the bioactive glassobject with bioactive glass particles delivered using an air abrasionsystem, wherein the bioactive glass object forms part of a medical orsurgical implant.
 3. A method of cutting a bioactive glass object whichcomprises contacting the bioactive glass object with bioactive glassparticles delivered using an air abrasion system, wherein the bioactiveglass object is formed from more than one bioactive glass.
 4. A methodof cutting a bioactive glass object which comprises contacting thebioactive glass object with bioactive glass particles delivered using anair abrasion system, wherein the bioactive glass particles comprises asource of SiO₂ or Si(OH)₂, and a source of CaO or P₂O₅.
 5. A method ofcutting a bioactive glass object which comprises contacting thebioactive glass object with bioactive glass particles delivered using anair abrasion system, wherein the bioactive glass particles furthercomprises at least one hardening agent and/or at least one softeningagent.
 6. A method according to claim 5 wherein the softening agent isselected from Na, K, Ca, Mg, B, Al, P, N, F and the hardening agent isTiO₂.
 7. A method of cutting a bioactive glass object which comprisescontacting the bioactive glass object with bioactive glass particlesdelivered using an air abrasion system, wherein the bioactive glassparticles comprises 1 to 100% SiO₂ or Si(OH)₂, 0–60% CaO, 0 to 60% P₂O₅,0 to 45% Na₂O, 0 to 45% K₂O and 0 to 40% MgO.
 8. A method of cutting abioactive glass object which comprises contacting the bioactive glassobject with bioactive glass particles delivered using an air abrasionsystem, wherein the bioactive glass particles are obtainable by Sol-Gelmethod.
 9. A method according to claim 8, wherein the bioactive glassparticles comprises 44 to 86 weight % SiO₂, 4 to 46 weight % CaO and 3to 15 weight % P₂O₅.
 10. A method to claim 8, wherein the bioactiveglass particles comprises 58 weight % SiO₂, 33 weight % CaO and 9 weight% P₂O₅.
 11. A method of cutting a bioactive glass object which comprisescontacting the bioactive glass object with bioactive glass particlesdelivered using an air abrasion system, wherein the bioactive glassparticles are obtainable by the Melt method.
 12. A method according toclaim 11, wherein the bioactive glass particles comprises 47 to 51weight % SiO₂23 to 25 weight % CaO, 23 to 25 weight % Na₂O and 0 to 6weight % P₂O₅.
 13. A method according to claim 11, wherein the bioactiveglass particles comprises (by weight): SiO₂—45% Na₂O—24.5% CaO—24.5%P₂O₅—6%.
 14. A method of cutting a bioactive glass object whichcomprises contacting the bioactive glass object with bioactive glassparticles delivered using an air abrasion system, wherein the bioactiveglass particles have a Vickers Hardness of at least that of thebioactive glass object.
 15. A method of cutting a bioactive glass objectwhich comprises contacting the bioactive glass object with bioactiveglass particles delivered using an air abrasion system, wherein thebioactive glass particles are substantially non-spherical.
 16. A methodof cutting a bioactive glass object which comprises contacting thebioactive glass object with bioactive glass particles delivered using anair abrasion system, wherein the bioactive glass particles aresubstantially spherical.
 17. A method of cutting a bioactive glassobject which comprises contacting the bioactive glass object withbioactive glass particles delivered using an air abrasion system,wherein the bioactive glass particles have a diameter of from 10 μm to500 μm.
 18. A bioactive glass surgical or dental implant cut accordingto a method of cutting a bioactive glass object which comprisescontacting the bioactive glass object with bioactive glass particlesdelivered using an air abrasion system.